Guide · Concrete
How Much Rebar Do I Need for a Concrete Slab?
A 10×10-foot residential slab reinforced with #4 (½″) rebar on an 18-inch grid needs 16 bars — 8 running each way — totaling 160 lineal feet and weighing about 107 pounds. Add 10% for lap splices and cutting waste, and buy roughly 120 pounds, which is nine 20-foot sticks.
Below is the bar-counting method, a chart of rebar needs for common slab sizes, a spacing comparison, and the weight math for different bar sizes. To skip the arithmetic, use our free rebar grid calculator — enter your slab dimensions and spacing and it returns bar counts, total footage, and weight.
Bar counts, lineal feet, and weight for your exact slab — free, no sign-up.
Open the rebar calculator →#4 rebar on an 18″ grid. With 10% for laps and waste: ~120 lb — nine 20-ft sticks.
The math, step by step
= (120 ÷ 18) + 1 = 7.67 → 8 bars
Total feet = 8 bars × 2 directions × 10 ft = 160 ft
Weight = 160 × 0.668 lb/ft (#4) = 106.9 → ~107 lb
- Convert the slab to inches. 10 ft = 120 inches per side.
- Count bars per direction. 120 ÷ 18 = 6.67, plus 1 = 7.67 — round up to 8 bars. The “+1” accounts for the bar at the starting edge; without it you’d be short one bar every time.
- Double it for the grid. A slab mat runs bars both directions: 8 × 2 = 16 bars, each 10 feet long.
- Total the lineal feet. 16 × 10 = 160 feet of bar.
- Convert to weight. #4 bar weighs 0.668 lb per foot: 160 × 0.668 = 106.9 → ~107 lb.
- Add 10% for laps and waste. Bars longer than a stick get spliced with a 24–30″ overlap, and cuts produce offcuts: 107 × 1.10 = 117.7 → order ~120 lb.
- Convert to sticks. Rebar is sold in 20-foot lengths: 160 ÷ 20 = 8 sticks, plus one for the laps → 9 sticks of #4.
Rebar counts for common slab sizes
All figures assume #4 bar on an 18″ grid, both directions, with the “+1” edge bar included.
| Slab size | Bars | Lineal ft | Weight (#4) | Order (+10%) |
|---|---|---|---|---|
| 10 × 10 ft | 16 | 160 | 107 lb | ~120 lb (9 sticks) |
| 12 × 12 ft | 18 | 216 | 144 lb | ~160 lb (12 sticks) |
| 12 × 16 ft | 21 | 288 | 192 lb | ~215 lb (16 sticks) |
| 10 × 20 ft | 23 | 310 | 207 lb | ~230 lb (18 sticks) |
| 20 × 20 ft | 30 | 600 | 401 lb | ~445 lb (33 sticks) |
Stick counts round up from (lineal ft × 1.10) ÷ 20. Suppliers sell full 20-ft sticks — you can’t buy a fraction of one.
What grid spacing should I use?
Eighteen inches on center each way is the workhorse default for residential slabs — patios, shed floors, walkways — using #4 bar. Here’s how spacing changes the same 10×10 slab:
| Grid spacing | Bars | Lineal ft | Weight (#4) |
|---|---|---|---|
| 12″ on center | 22 | 220 | 147 lb |
| 18″ on center | 16 | 160 | 107 lb |
| 24″ on center | 12 | 120 | 80 lb |
Tighter spacing = more steel = stiffer slab. Driveways and slabs carrying heavy loads often go to 12″; check what your local code requires before choosing.
Rebar sizes and weights
Bar size is given as a number where each unit equals ⅛″ of diameter: #4 is &frac48;″ = ½″. Weight per foot is fixed by the steel industry, so the math is exact once you know the footage:
| Bar size | Diameter | Weight per foot | 160 ft weighs |
|---|---|---|---|
| #3 | ⅜″ | 0.376 lb/ft | 60 lb |
| #4 | ½″ | 0.668 lb/ft | 107 lb |
| #5 | ⅝″ | 1.043 lb/ft | 167 lb |
#4 is the common residential slab choice. #5 shows up in heavier structural work; #3 in light-duty applications and stirrups.
Laps, chairs, and ties
Three small items complete the order beyond the bars themselves. Lap splices: whenever a run needs more than one 20-foot stick, overlap the bars 24–30 inches for #4 (a common rule of thumb) and wire the splice — this is what the 10% allowance pays for. Chairs or dobies: these little supports hold the mat at the right height — typically mid-depth of the slab — so the steel doesn’t end up lying on the ground doing nothing; space them roughly 4 feet apart each way. Tie wire: twist-tie the intersections (every other one is plenty) so the grid doesn’t shift while the concrete is placed. And keep every bar 2–3 inches clear of the slab’s edges and bottom — steel that pokes through the surface rusts, and rust expands and cracks concrete.
Rebar vs. wire mesh
For light-duty slabs — shed floors, sidewalks, small patios — welded wire mesh (the 6×6 sheets) is cheaper, faster to lay, and perfectly adequate as a planning default. Step up to rebar for driveways, garage floors, and any slab carrying real loads or spanning questionable soil. You generally need one or the other, not both. Either way, size the concrete order itself with our concrete calculator once the reinforcement is decided.
How we calculate
Bar counts use (slab dimension in inches ÷ spacing) + 1 per direction, rounded up — the +1 covers the starting edge bar. Total lineal feet multiply bar counts by bar lengths; weight multiplies footage by the industry-standard weight per foot (#4 = 0.668 lb/ft, #3 = 0.376, #5 = 1.043). The 10% allowance covers lap splices (24–30″ overlaps as a rule of thumb) and cutting waste. Spacing, cover, and lap guidance here are planning rules of thumb, not engineering specifications — structural slabs should follow stamped plans and local code.
Reviewed September 2026 · TrueMeasure
Frequently asked questions
How far apart should rebar be spaced in a concrete slab?
18 inches on center each way is the common residential default for #4 rebar. Heavier loads may call for 12-inch spacing. Always confirm against your local code — it is the final word.
Can I use #3 rebar instead of #4 in a slab?
#3 bar (3/8″, 0.376 lb/ft) is lighter and appears in some residential work, but #4 (1/2″, 0.668 lb/ft) is the more common slab choice. Follow your plans or local code rather than substituting on your own.
How much should rebar overlap at splices?
Plan on 24 to 30 inches of lap for #4 bars as a rule of thumb — this is exactly what the 10 percent waste allowance in the estimate covers.
Do I need both rebar and wire mesh in a slab?
Usually one or the other for residential slabs: rebar for driveways and heavier loads, welded wire mesh for light-duty slabs like sheds and sidewalks. Doubling up is generally unnecessary.
How many 20-foot sticks of rebar for a 10×10 slab?
Nine sticks of #4. The slab needs 160 lineal feet (160 ÷ 20 = 8 sticks), plus one more stick to cover lap splices and cutting waste.
How deep should rebar sit in a concrete slab?
Typically at mid-depth of the slab, or per your plan, with 2 to 3 inches of concrete cover from the edges and bottom so the steel stays protected from moisture. Treat this as a planning rule of thumb, not an engineering specification.